1. Field of the Invention
The present invention relates to a method of preparing a large-current printed circuit board. For instance, it relates to a method of preparing a large-current printed circuit board device in which a busbar is provided at a printed circuit board so as to form a large current electric circuit.
2. Discussion of Background
FIG. 7 is a cross-sectional view showing a conventional large-current circuit board, e.g. a printed circuit board device in which a busbar is used so as to form a large current electric circuit, described in, for instance, Japanese Unexamined Patent Publication No. 159787/1990. In FIG. 7, reference numeral 1 designates a busbar which is made of a slender copper plate of 12 mm wide and 1.2 mm thick, for instance. Numeral 2 designates a fastener made of an electric conductive material such as copper, brass or the like. Numeral 4 designates a lead terminal fastened to a circuit element by means of a screw, the lead terminal having a threaded hole 4a at a portion near its free end. Numeral 5 designates a screw and numeral 6 designates a printed circuit board.
First, a process of preparing the large-current printed circuit board having the construction described above, will be described. A through-hole is formed at a predetermined position in the printed circuit board 6 so that the diameter of the through-hole is about 0.1 mm-0.5 mm larger than the outer diameter of the fastener 2. Then, the fastener 2 is inserted to the through-hole so that a bottom flange formed at its one end is engaged with a surface of the printed circuit board 6. A caulking operation is conducted to the fastener 2 with use of a tool for exclusive use so that a radial extension 2a is formed at an intermediate of the fastener 2, whereby the fastener is fixed to the printed circuit board 6. Then, an insertion hole of the busbar 1 is aligned to the bore of the fastener 2, and the screw 5 is engaged with the threaded hole 4a of the lead terminal 4, whereby the busbar is fixed to the circuit element 3 to thereby form a large-current printed circuit board to which the printed circuit board 6 is attached.
The detailed description will be made as to the fastener 2. FIGS. 8a and 8b are respectively cross-sectional views showing a conventional fastener 2 wherein FIG. 8a shows such fastener having a portion 2a before a caulking operation and FIG. 8b shows the fastener in a state that the fastener is fastened to a printed circuit board 6. The fastener 2 is made of a metallic material and comprises a cylindrical body having an uniform outer diameter and a bottom flange 2b at an end of the cylindrical body, the outer diameter of the flange 2b being larger than the outer diameter of the cylindrical body. The wall thickness of the cylindrical body is formed so that it is stepwisely reduced from the side of the flange 2b. Namely, the wall thickness of the cylindrical body near the flange is thinner, and the wall thickness remote from the flange 2b is thicker.
The fastener 2 is inserted into a through-hole formed in the printed circuit board 6 so that the flange 2b is engaged with a surface of the board 6. Then, both ends of the fastener 2 is compressed toward its center in the axial direction by using a tool for exclusive use, whereby a portion having a thinner wall thickness is expanded outwardly to thereby form a radial extension or a caulking portion 2a so that the fastener is fixed to the printed circuit board 6. Thus, in the conventional large-current printed circuit board, the fastener 2 is fixed to the printed circuit board 6 by means of the radial extension 2a and the flange 2b, and a busbar 1 and a circuit element 3 are connected to the fastener 2 with use of a screw, whereby each of the parts is strongly and mechanically connected to the printed circuit board 6.
FIG. 9 shows in cross-section another conventional large-current printed circuit board. In FIG. 9, a through-hole and a land portion 6a are formed in a printed circuit board 6a in which a fastener 2 is inserted. An electric circuit is printed on the board so that the circuit is suitably connected to the land portion 6a. A busbar 1 and a circuit element 3 are fixed to the fastener 2 by engaging a screw 5 with a threaded hole 4a which is formed at the upper part of the fastener 2. Further, a busbar 1 is interposed between the lower surface of the printed circuit board 6 and the flange 2b of the fastener 2, which is formed at the lower portion of it.
In the conventional method of preparing a large-current printed circuit board, there is a problem of a change of the material for the circuit board by aging when the printed circuit board is continuously used for a long time. The conventional printed circuit board 6 is mainly made of a glass epoxy resin. Accordingly, when a large current is continuously supplied for a long time, there causes heat deterioration in the entire or a local portion of the printed circuit board due to a temperature rise at a current conducting portion to thereby cause problems of reduction of thickness of the board due to the deterioration of the epoxy resin and reduction of the creeping characteristic. These result in the weakening of the fitting strength of the fastener 2 to the printed circuit board 6 and the delamination or the measling of the printed circuit board at a portion near the fixing part. Further, the resistance of contact between the busbar 1 or the busbar 1a to the fastener 2 becomes high because the fitting strength becomes weak, with the result of causing electric and thermal defects. Further, in case that a printed circuit is formed on the printed circuit board 6, the resistance of contact between the land portion 6a and the fastener 2 becomes high to thereby cause electric and thermal defects. Further, when the fastener 2 is inserted in the printed circuit board 6 and the both ends of the fastener 2 is compressed toward the center in the axial direction to thereby fix the fastener 2 to the printed circuit board 6 by caulking, a portion having a thinner wall thickness of the fastener 2 is expanded outwardly in such a manner that an excessive force is locally applied to corner portions of the through-hole and the land portion 6a so that parts of the through-hole and the land portion 6a may be broken. In addition, since the radial extension 2a has a curved shape, the surface of contact of the radial extension 2a is not always flat and the fitting force to the flange 2b is uneven, whereby the printed circuit board 6 may be curved after the caulking operation.